Acetone is treated with C2H5MgBr and then H3O+; the product is heated with conc. H2SO4 to give the major (Saytzeff) alkene; this alkene is then treated with BH3/THF followed by H2O2/NaOH. The final major product is

JEE Advanced levelSingle correct optionDifficulty 3 of 3ap015

Acetone is treated with $C X_{2} H X_{5} MgBr$ and then $H X_{3} O X^{+}$ ; the product is heated with conc. $H X_{2} SO X_{4}$ to give the major (Saytzeff) alkene; this alkene is then treated with $BH X_{3}$ /THF followed by $H X_{2} O X_{2}$ / $NaOH$ . The final major product is

Step 1 gives 2-methylbutan-2-ol, and dehydration (Saytzeff) gives the trisubstituted alkene 2-methylbut-2-ene,

(CH X_{3}) X_{2} C = CH - CH X_{3}

. Hydroboration places boron on the less substituted alkene carbon (the

= CH X^{-}

carbon), so OH ends up on C-2 of the butane chain bearing a methyl at C-3: 3-methylbutan-2-ol.